Chemical and nonchemical stimuli during breath holding in divers are not independent

In the breath-hold model described by S. Godfrey and E. J. M. Campbell (Respir. Physiol. 5: 385-400, 1968), chemical and nonchemical stimuli are independent. Because these two factors are time dependent, the effect of each could not be measured by breath-holding time (BHT). The aim of this study is to dissociate chemical and nonchemical stimuli and to assess the effects of BHT and PCO2 on respiratory center output by measurement of occlusion pressure (P0.1) and mean inspiratory flow (VI). Nine well-trained divers (age 36.5 +/- 5.0 yr) took part in the study. Each subject had to hold his breath at 75% of vital capacity for 30, 50, and 70 s of BHT. Before each breath hold, the subject inspired successively two vital capacities of the same CO2-O2 gas mixture. P0.1 and VI were measured during the first reinspiration after the breath hold. For the same BHT, we observed good linear relationships between P0.1 or VI and alveolar PCO2. The slopes of these relationships increased with BHT. For alveolar PCO2 of > 50 Torr, P0.1 increased linearly with BHT. These results indicate that, during breath holding, chemical and nonchemical stimuli acted linearly on respiratory motoneuron activity, but they were not independent.